A hybrid vehicle which has both of an engine and a rotating electrical machine as a power source for moving is known. The hybrid vehicle like this has an electricity storage apparatus (in other words, an electrical power source or a battery) and a voltage converter which converts an output voltage of the electricity storage apparatus (for example, a booster converter which boosts or steps down the output voltage). When the output voltage is boosted by the voltage converter, the rotating electrical machine is driven by a relatively high voltage. Thus, a high power or a high efficiency (namely, a reduction of a loss) of the rotating electrical machine is achieved.
What we call a chopper type boost converter is often used as the voltage converter (for example, see Patent Literatures 1 to 3). Namely, a voltage converter in which an upper switching element at a high potential side and a lower switching element at a low potential side are connected in series is often used as the voltage converter.
Here, the Patent Literature 1 proposes a technology for setting a lower limit value of a duty ratio which represents a rate of a period during which the upper switching element is turned on (namely, the period during which the upper switching element is turned on/(the period during which the upper switching element is turned on+a period during which the lower switching element is turned on)) in order to maximize an electrical power which is outputted from the electricity storage apparatus via the voltage converter. Therefore, according to the technology which is disclosed in the Patent Literature 1 when the duty ratio which is calculated to convert the output voltage of the electricity storage apparatus into a desired voltage is lower than the lower limit value, the duty ratio which is actually used is limited to the lower limit value. Moreover, the Patent Literature 1 proposes a technology for setting an upper limit value of the duty ratio.
In addition, another example of the hybrid vehicle which has the voltage converter is disclosed in the Patent Literature 2 and the Patent Literature 3. Specifically, the Patent Literature 2 discloses a hybrid vehicle which greatly changes an output current of the electricity storage apparatus by changing the duty ratio more greatly than an aspect of changing the duty ratio from a duty ratio which depends on a first command value to a duty ratio which depends on a second command value, when the output current of the electricity storage apparatus is changed from the first command value to the second command value. Moreover, the Patent Literature 3 discloses a hybrid vehicle which changes the lower limit value of the duty ratio on the basis of a SOC (State Of Charge).